One of the key problems in planetary system formation is understanding how rapidly, and over what time interval Jovian planets can form. Dust in the protoplanetary disk is critical in planetesimal formation, but it is the gas which produces giant planets, and which is essential for their migration. However, compared to data on the circumstellar dust, information on the gas component is sparse, especially in the planet-formation zone. This severely limits our ability to put observational constraints on giant planet formation, except to note that the process must be largely complete by 12 Myr, given the paucity of Herbig Ae or classical T Tauri stars older than 10-12 Myr. In the FUV, photo-excited molecular hydrogen transitions have the requisite contrast to the stellar photosphere, accretion shock, and reflection nebulosity, and can be traced 50-100 AU from the exciting stars in both envelopes and outflow cavities and protoplanetary disks. Central disk cavities, an expected consequence of planet formation, larger than 0.1" are directly detectable in HST FUV spectra, while smaller cavities may be detected by comparison with protoplanetary disks which are still accreting onto their stars. We propose augmenting existing HST coronagraphic imagery of 6 Herbig Fe and T Tauri disks with ACS Solar-Blind Channel Lyman alpha imagery and slitless spectroscopy simultaneously sampling the disk in molecular hydrogen and small-grain reflection nebulosity. These data will be used to quantify the amount of vertical stratification in these disks, to map the mass-loss geometry from the star, and to determine whether removal of molecular material preceeds, lags, or is contemporary with clearing of the dust.

Program Stars: We will obtain ACS SBC F122M (Lyman alpha) images and SBC PR130L spatially-resolved spectra of each of our program science targets and one white dwarf. The white dwarf will be used as a deep PSF comparison object. Our program stars have well-studied disks, archival optical and NIR coronagraphic imagery. The stars have known inclinations, optical outflow data, and are minimally affected by remnant envelope material. We are also requesting a deep PSF observation of an ACS SBC spectrophotometric standard, currently HS 2027 +0651. For each target, we are requesting the equivalent of a full orbital visibility for F122M imagery, to permit easy comparison with the Herbig Ae stars HD 100453 and HD 169142 which are to be observed under HST-GO-10764. The remaining time will be spent upon the PR130L spectroscopy, with the equivalent of 2 orbits for all targets except HD 135344 and the white dwarf. All of the observations will be obtained with the ACS small-box dither pattern to permit us to monitor the sources and any low-mass companions for flaring. HD 135344: This F4Ve star has an archival FUSE spectrum (Q306020100) showing the faint (5E-15 erg/cm/cm/s/A) continuum and emission-line spectrum expected for a mid-F star with very little accretion. Based on the FUSE data the dominant emission-line source should be chromospheric and transition region features in the ACS spectral range. The stellar photosphere at the long wavelength end of the PR130L response will be the dominant signal source, but even when conservatively modelled as F2V (V=8.61), there are no warnings for the star. In 2ks, the integrated counts are above 120,000 c/pixel from 1750-1950 A, so dithering will be essential to avoid loss of data. For the F122M imagery, we have a) scaled from HD 169142, an A8Ve Herbig Ae star with low current accretion rate, and b) scaled from the FUSE C III 1176 profile. Both approaches result in a peak count rate of 20-25 c/s, with no MAMA bright object warnings. HD 135344 has a foreground F2V star (CD -36 10010, V=7.9, F2V at 15H15M48.95S -37D08'56.11") about 22" to the NNE of our science target. In the event of a bad manuever to HD 135344, this object may end up in the SBC FOV, but does not exceed either the global or local bright object limits. In the event that it is in the SBC, we will use it as a comparison object. CQ Tau: CQ Tau is a UXOR, a large amplitude light and color variable PMS star in the optical (V=9.7 to 11.2). However, in the FUV, the continuum is near 1E-14 erg/cm/cm/s/A in repeated IUE observations, ranging from optical maximum to optical minimum light. These data suggest the star may be obscured by the inner disk and visible only via scattered light in the FUV. There are no MAMA bright limit issues for this star. CY Tau: This is the UV-brightest of our program T tauri stars, and has an archival STIS E140M (0.2x0.2" aperture). The strongest line in the UV spectrum is Lyman alpha, with a peak flux of 6E-14 erg/cm/cm/s/A. C IV, He II, and C III 1176 are the next strongest features with peak fluxes near 3-4 E-14 erg/cm/cm/s/A. The continuum is near 1E-15 erg/cm/cm/s/A. The ACS SBC F122M image will be dominated by Lyman alpha and will have a peak count rate of 0.82 c/p/s, yielding S/N=106 in 2ks. The PR130L spectrum will have the highest count rate at C IV, 2.9 c/p/s, for a total signal near 30kcounts in 2 ks. The spectrum in the H_2 bands will have approximately 6Kcounts/pixel in 2ks. The STIS data indicate that CY Tau is isolated. STIS coronagraphic imagery have revealed an HH knot at PA=65.2D which was 2" from the star in 2001. This knot and any more extended jet material should be visible in the F122M imagery, but should not appreciably contaminate the spectral image. GM Aur: This star has a STIS G140L spectrum. From 1420-1510 A, the spectrum has a faint continuum, with fluxes near 3E-15 erg/cm/cm/s/A, with lines a factor of 2 brighter. At the resolution of the PR130L, the signal is 5E-15 erg/cm/cm/s/A, with the signal in the reflection nebulosity band near 2E-15 erg/cm/cm/s/A. The strongest emission lines in the FUV are C IV (peak 5E-14 erg/cm/cm/s/A) and He II (3E-14 erg/cm/cm/s/A). For ACS, the ETC indicates a peak count rate at C IV of 4.37 c/s. In the H_2 band, the on-source S/N is ~73: binning over the band means that, absent airglow, we will expose the wings of the PSF to 30 counts/band 1" from the star. GM Aur has fluorescent H_2 emission a factor of 2 above this level in the STIS spectrum (at 0.5"). In a high airglow environment, we will be able to detect the circumstellar emission only to 0.5" (which is still 70 AU from the star). The F122M image will yield an on-source S/N of 51 under low background conditions, and 34 under high airglow conditions in 300s. NICMOS saw a ribbon of nebulosity to the NE of the star which may be a molecular cloud rim. This structure will fall within the ACS field of view. AA Tau and DL Tau: GM Aur is brighter than our other program stars, DL Tau, and AA Tau, both in the on-star spectrum (judging by the IUE data), and in terms of its reflection nebulosity in the NIR. We therefore anticipate no MAMA bright object issues with any of these stars. Serendipitous Science: AA Tau, DL Tau, and CY Tau are all known to drive microjets with HH knots (Grady et al. 2006) which will be within the ACS SBC field of view. The DL Tau field also contains 3 other bright, red sources within 12" of DL Tau. These are expected to be additional, lower mass T Tauri stars. N/A Identification of low-surface brightness extended emission in either Lyman alpha or the fluorescent molecular hydrogen transitions requires knowledge of the instrumental point spread function. Current calibration of the ACS SBC is optimized for routine point source spectral extraction and lacks the contrast we require. As a result, we have requested 2 orbits for observation of a PSF template object, baselined as HS 2027 +0651, with the observations carried out with the same aperture, dither pattern and relative distribution of exposure time as for our science targets. STAR FORMATION 12 false 0.0 false false false false false AVAILABLE STAR T TAURI STAR PROTOPLANETARY DISK PRE-MAIN SEQUENCE STAR JET This object was generated by the targetselector and retrieved from the SIMBAD database. C IV 1.4E-14 erg/cm/cm/s/A, FUV continuum 3-5E-15 erg/cm/cm/s/A HBC-63 GSC-01833-00851 J2000 04 34 55.4200 +24 28 53.30 J2000 STAR JET PRE-MAIN SEQUENCE STAR PROTOPLANETARY DISK T TAURI STAR This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 04 33 39.0800 +25 20 38.10 J2000 STAR JET PRE-MAIN SEQUENCE STAR PROTOPLANETARY DISK T TAURI STAR This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 04 17 33.7200 +28 20 46.80 J2000 STAR HERBIG AE/BE JET PRE-MAIN SEQUENCE STAR PROTOPLANETARY DISK This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 05 35 58.4670 +24 44 54.09 J2000 STAR JET PRE-MAIN SEQUENCE STAR PROTOPLANETARY DISK T TAURI STAR This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 04 55 10.9800 +30 21 59.40 J2000 STAR HERBIG AE/BE PRE-MAIN SEQUENCE STAR PROTOPLANETARY DISK This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 15 15 48.4390 -37 09 16.03 J2000 STAR WDO This object was generated by the targetselector and retrieved from the SIMBAD database. 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